Initial commit: dashdrm-mp4ff plugin

Add a Streamlink plugin (plugin/dashdrm_mp4ff.py) to support DRM-protected MPEG-DASH streams with optional mp4ff-decrypt integration. Include README with usage and installation notes, an interactive helper (gen.py) that reads vault.json and prints Streamlink commands, a vault.json example, pyproject.toml (declares inquirerpy dependency), .python-version, and .gitignore. This provides the plugin, packaging metadata, and tooling needed to run and test DASH-DRM streams locally.

README.md

@@ -0,0 +1,75 @@
+# dashdrm-mp4ff
+
+Custom Streamlink plugin for DRM-protected MPEG-DASH streams.
+
+This project is based on [titus-au/streamlink-plugin-dashdrm](https://github.com/titus-au/streamlink-plugin-dashdrm), which itself is based on Streamlink's [`dash.py`](https://github.com/streamlink/streamlink/blob/master/src/streamlink/stream/dash/dash.py).
+
+## Install `mp4ff-decrypt`
+
+```bash
+go install github.com/Eyevinn/mp4ff/cmd/mp4ff-decrypt@latest
+```
+
+Make sure the Go bin directory is in your `PATH`, then verify:
+```bash
+mp4ff-decrypt -h
+```
+
+If you do not want to use `mp4ff-decrypt`, you can still use the plugin without `--dashdrm-mp4ff-use-mp4ff`.
+
+## Usage
+
+Run Streamlink with the local plugin directory:
+```bash
+streamlink --plugin-dirs "plugin" "dashdrm://https://example.com/live.mpd" best
+```
+
+With `mp4ff-decrypt` and `kid:key` pairs:
+
+```bash
+streamlink \
+  --plugin-dirs "plugin" \
+  --dashdrm-mp4ff-use-mp4ff \
+  --dashdrm-mp4ff-decryption-key "kid1:key1,kid2:key2,kid3:key3" \
+  "dashdrm://https://example.com/live.mpd" \
+  best
+```
+
+Notes:
+
+- Keys can be provided as raw keys or as `kid:key`.
+- When `kid:key` pairs are provided, the plugin parses the manifest and matches each track with the correct key.
+- The plugin file is [`plugin/dashdrm_mp4ff.py`](./plugin/dashdrm_mp4ff.py).
+
+## `gen.py`
+
+[`gen.py`](./gen.py) is a small helper that reads channels from `vault.json`, lets you pick one interactively, and prints the full Streamlink command.
+
+Install the helper dependency:
+
+```bash
+uv sync
+```
+
+Create your own `vault.json` from [`vault.json.example`](./vault.json.example), then run:
+```bash
+python gen.py
+```
+
+The expected structure is:
+```json
+{
+  "channels": [
+    {
+      "id": "channel-id",
+      "number": "1",
+      "name": "Channel name",
+      "mpd_url": "https://example.com/live.mpd",
+      "keys": [
+        "kid1:key1",
+        "kid2:key2"
+      ]
+    }
+  ]
+}
+```